For more than a century, it has been known that certain materials are sensitive to actinic radiation and, upon exposure to such radiation, form latent images capable of being subsequently developed into a useful visible image. Almost exclusively, commercial application of these radiation sensitive materials has been the domain of silver halides which exhibit superior sensitivity to light over other radiation sensitive materials, some of which have been known for as long as silver halides have been in use. Such superior sensitivity has made silver halides more practical for use in cameras and other photographic equipment since they can be utilized in low light situations, or in situations where the mechanical characteristics of a camera (or other exposure means) would interfere with an optimum exposure.
Because it is desirable to improve the sensitivity of silver halide crystals, and hence the photographic elements in which they are contained, photographic chemists have over the years attempted to vary the processes for making, or the components within, silver halide emulsions. Typically, the process of making photographic emulsions has consisted of the following steps:
1. The formation of a dispersion of microcrystals (grains) of silver halide in a solution of protective colloid. PA1 2. Physical ripening to achieve the desired grain size. PA1 3. The freeing of the dispersion from excess soluble salts by washing or by coagulation followed by redispersal in a salt free medium. PA1 4. A heat treatment, often in the presence of sensitizing agents, to increase light sensitivity. The treatment is variously termed after-ripening, second ripening, chemical ripening, or digestion. PA1 5. Addition of agents that confer the desired properties, such as sensitizing dyes, antifoggants and stabilizers, and hardeners.
It is during these steps that photographic chemists have added additional components to the emulsion, or varied the conditions under which the emulsion is formed, in order to optimize certain emulsion characteristics. One such characteristic is contrast, and it is generally desired that contrast be maximized.
High contrast is desirable because it is directly related to the appearance of sharpness in an emulsion. One measure of contrast is the sharpness of the toe section of an emulsion's D-log E curve. By sharpness of the toe section, it is usually meant the relative density of the toe section. For instance, a sharp toe corresponds to a relatively low (small) toe density, and a soft toe corresponds to a relatively high (large) toe density. Generally, the point at which toe density is measured corresponds to 0.3 log E fast of the speed point, which is defined as the point on the D-log E curve where density equals 1.0.
It has been known that improvements in the contrast of photographic emulsions can be obtained by employing various transition metals as either dopants or grain surface modifiers. Dopants are substances added to an emulsion during silver halide precipitation which typically become incorporated within the internal structure of the silver halide grains. Grain surface modifiers, by contrast, are added post-precipitation, thus becoming associated with the surface of the silver halide grains rather than their internal structure.
Recently, emulsion dopants have been described which comprise transition metal complexes having nitrosyl or thionitrosyl ligands. European Patent Applications 0325235 and 0457298 disclose the use of one such complex, namely potassium ferric pentacyanonitrosyl. A second type of dopant, rhenium nitrosyl or rhenium thionitrosyl is disclosed in U.S. Pat. No. 4,835,093; and a third, dicesium pentachloronitrosyl osmate, is disclosed in U.S. Pat. No. 4,933,272. The third is known, in certain instances, to increase the contrast of photographic emulsions. It does so, however, at the expense of a corresponding decrease in sensitivity. Sensitivity (or "speed") of an emulsion is usually defined as the reciprocal of the relative amount of light in Log E.times.100 to produce 1.0 density.
It has been recognized in the art that photographic sensitivity (speed) can be increased by adjusting the pH and/or the pAg of a silver halide emulsion. It has also been known that enhanced photographic sensitivity can be obtained by the addition of certain types of chemical sensitizers. Several types of sensitizers have been identified. The most prevalent are gold and sulfur compounds, both of which are thought to enhance emulsion speed by forming electron or photohole traps on the silver halide crystal surface.
Sensitization has also been accomplished by the addition of other transition metals. Specifically, platinum salts have been used, although sensitization with such salts is strongly retarded by gelatin. In addition, iridium salts and complex ions of rhodium, osmium, and ruthenium have been used as chemical sensitizers. The overall effect of these metals on sensitivity appears to be dependant upon their valence state.
The ability of certain alkynylamines to produce increases in speed when incorporated into photographic emulsions as grain surface modifiers (post-precipitation) has been described in prior U.S. Pat. Nos. 4,378,426 and 4,451,557.
Reduction sensitization is another means by which sensitivity enhancement can be obtained. Known reducing agents include stannous chloride, ascorbic acid (as in European Patent Applications 0369491 and 0369424) and dimethylamine borane (U.S. Pat. Nos. 4,150,093 and 3,782,959). However, the use of such sensitizers, as well as other types of sensitizers described above, often causes an indiscriminant, and undesirable, reduction of silver ions to silver atoms. The silver atoms give rise during development to a generally or locally developed density that is not associated with the action of the image forming exposure. This density is typically termed fog and it is best measured as the minimum density on the emulsion's D-Log E curve (D.sub.min).
Fog is undesirable because it is an indication that the discrimination between the photographic image and the unexposed area is reduced. Thus, the quality of the image obtained is less than optimal. For silver chloride emulsions, which are more prone to being reduced than emulsions containing silver bromide or silver iodide (or mixtures thereof), this problem is even more severe.
A problem closely linked with fog is the tendency for silver halide emulsions, whether chemically sensitized or not, to exhibit unwanted sensitometric changes--such as an increase in speed--during their storage (aging). The ability of an emulsion to be resistant to these storage changes is called the emulsion's stability (or "keeping ability").
To improve an emulsion's stability, and to control the formation of fog, it has been known to add to the emulsion chemicals known as antifoggants and stabilizers. Because some of these compounds function in both capacities often only one compound is added.
Known antifoggants include the mercaptotetrazoles, tetraazaindenes, benzothiazolium salts, and the thiosulfonates. The thiosulfonates, in particular, have been the subject of much study in recent years. Examples of their uses can be found in the following references: European Patent Applications 0369424, 0435355, 0371338, 0368304, 0348934, U.S. Pat. No. 5,079,138, German Patent 3,908,835, JO3,237,453 and JP4,083,241, as well as in S. Gahler in Veroff. wiss. Photolab. Wolfen X, 63(1965), and in Z. Wiss. Photo. 63 133(1969).
Thiosulfonates, and especially their alkali salts, suffer the disadvantage of being particularly prone to decomposition. Such decomposition, if not carefully controlled, often gives rise to unacceptable photographic emulsions exhibiting variable speed and high fog.
One method of improving the antifoggant/stabilizer effects of thiosulfonates is to incorporate them into an emulsion along with an amount of sulfinate salt. U.S. Pat. No. 2,394,198 discloses an improvement in the effects of sulfinate salts by the addition to silver halide emulsions amounts of thiosulfonates. Japanese Kokai 3-208041 discloses the use of thiosulfonates and sulfinates during grain formation to make the antifogging effect more obvious. European Patent Application 0358170 discloses the use of thiosulfonates and sulfinates in a direct positive emulsion.
Although the combination of thiosulfonates and sulfinates is known, emulsions containing this combination lack the desired degree of sensitivity and resistance to storage changes. It is therefore desired to provide photographic emulsions containing thiosulfonates and sulfinates which exhibit high sensitivity and resistance to storage changes. Furthermore, it is desired that such emulsions exhibit their characteristics without exhibiting any significant loss in contrast.